Adsorption of Aqueous Metal Ions on Oxygen and Nitrogen

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Adsorption of Aqueous Metal Ions on Oxygen and Nitrogen Functionalized Nanoporous Activated Carbons B. Xiao and K. M. Thomas* Northern Carbon Research Laboratories, School of Natural Sciences, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K. Received November 22, 2004. In Final Form: January 26, 2005 In this study, the adsorption characteristics of two series of oxygen and nitrogen functionalized activated carbons were investigated. These series were a low nitrogen content (∼1 wt % daf) carbon series derived from coconut shell and a high nitrogen content (∼8 wt % daf) carbon series derived from polyacrylonitrile. In both series, the oxygen contents were varied over the range ∼2-22 wt % daf. The porous structures of the functionalized activated carbons were characterized using N2 (77 K) and CO2 (273 K) adsorption. Only minor changes in the porous structure were observed in both series. This allowed the effect of changes in functional group concentrations on metal ion adsorption to be studied without major influences due to differences in porous structure characteristics. The surface group characteristics were examined by Fourier transform infrared (FTIR) spectroscopy, acid/base titrations, and measurement of the point of zero charge (pHPZC). The adsorption of aqueous metal ion species, M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed have a linear relationship for the carbons with pHPZC e 4.15. Hydrolysis of metal species in solution may affect the adsorption of metal ion species and displacement of protons. In the case of basic carbons, both protons and metal ions are adsorbed on the carbons. The complex nature of competitive adsorption between the proton and metal ion species and the amphoteric character of carbon surfaces are discussed in relation to the mechanism of adsorption.

1. Introduction Emissions standards are becoming increasingly stringent for toxic heavy metals present in aqueous effluents, which must be removed, to minimize the environmental impact. Chemical precipitation or electrochemical methods can be used to remove aqueous heavy metal pollutants present in high concentrations. However, final trace concentrations of metal species not removed by these treatment procedures must also be removed, and this can be achieved by adsorption on activated carbon.1-5 Activated carbons have well developed porous structures and specific surface chemical properties which may be varied by chemical and heat treatment procedures. These materials are widely used in industry for the removal of many organic compounds from both liquid and gas phases. The surface chemistry of activated carbons involves both hydrophobic graphene layers and hydrophilic functional groups. Organic compounds are adsorbed on the former, whereas polar species are adsorbed on the latter. Chemical oxidation, which incorporates large quantities of oxygen functional groups on the surface of activated carbon, enhances the adsorption of aqueous metal cation species and modifies the selectivity of the activated carbon for these aqueous metal species.1-5 Oxygen functional groups are involved in the formation of surface complexes with aqueous metal species and ion exchange with displacement of protons.1,2,6 Aqueous metal ions have different affinities

for various functional groups such as carboxylic and phenolic groups on the carbon surface. Nitrogen functional groups can be incorporated into the carbon structure either by carbonization of nitrogen-rich precursors or by high temperature ammonia treatment. Pyridinic functional groups coordinate with aqueous metal species, but the metal ions are displaced at low pH due to competitive adsorption.1 Metal anionic species are adsorbed by a different mechanism; for example, the aurocyanide ion is mainly adsorbed on the graphene layers and as KAu(CN)2 ion pairs.7,8 Redox reactions involving the aqueous metal species have also been proposed.9,10 Adsorption from aqueous solution is complex and may involve electrostatic effects, ion exchange, and coordination with functional groups on amphoteric carbon surfaces.11 Several factors, such as surface charge and speciation of metal ions in solution, affect the adsorption of aqueous metal species on activated carbon. This leads to a dependence of the amount adsorbed on the point of zero charge (pHPZC), isoelectric point, and experimental conditions, such as ionic strength, pH, and species concentrations. Studies suggest that the adsorption of aqueous species onto a hydrated solid surface must overcome an extra energy barrier to complete the exchange of hydration spheres.12 The stability of surface complexes formed between metal ions and activated carbon depends on surface chemistry and porous structure characteristics.1,2,5,11 The adsorption of aqueous metal ions is also strongly influenced by the competition of different aqueous metal ions to occupy the

* Corresponding author. E-mail: [email protected]. (1) Jia, Y. F.; Xiao, B.; Thomas, K. M. Langmuir 2002, 18, 470. (2) Jia, Y. F.; Thomas, K. M. Langmuir 2000, 16, 1114. (3) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A.; Moreno-Castilla, C. Carbon 1994, 32, 93. (4) Rivera-Utrilla, J.; Ferro-Garcia, M. A. Adsorpt. Sci. Technol. 1986, 3, 293. (5) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24, p 221. (6) Carapcioglu, M. O.; Huang, C. P. Water Res. 1987, 21, 1031.

(7) Jia, Y. F.; Steele, C. J.; Hayward, I. P.; Thomas, K. M. Carbon 1998, 36, 1299. (8) Jia, Y. F.; Thomas, K. M. J. Phys. Chem. B 2004, 108, 17124. (9) Huang, C. P.; Blankenship, D. W. Water Res. 1984, 18, 37. (10) Lo´pez-Gonza´lez, J. D.; Moreno-Castilla, C.; Guerrero-Ruiz, A.; Rodriguez-Reinoso, F. J. Chem. Technol. Biotechnol. 1982, 32, 575. (11) Huang, C. P. In Carbon Adsorption Handbook; Cheremisinoff, P. N., Ellerbusch, F., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1978; p 281. (12) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 66.

10.1021/la047135t CCC: $30.25 © 2005 American Chemical Society Published on Web 03/19/2005

Adsorption of Aqueous Metal Ions

limited number of surface sites, which decreases the removal efficiency of activated carbon for the more toxic metals of interest.13 In this paper, the adsorption characteristics of Ca2+(aq), Cu2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) species on functionalized activated carbons were studied. The oxygen contents of the low (∼1 wt % daf) and high (∼8 wt % daf) nitrogen content carbon series varied over the range ∼2-22 wt % daf. There were only minor changes in the porous structure within a given series. This allowed the effects of functional group concentrations to be investigated independently of porous structure characteristics. The pH of the solution was not controlled by the addition of buffer solution because the ionic components of the buffer would interfere with the metal ion competitive adsorption process. The solution pH was measured as a function of metal ion adsorption to quantify the adsorption or displacement of protons during the adsorption process. The objective was to understand the mechanism of metal ion adsorption on activated carbons in relation to their amphoteric character and surface chemistry. 2. Experimental Section 2.1. Materials Used. Activated Carbon Used. Commercially available coconut shell derived carbon G prepared by physical activation using steam at 1173 K was used in this study. This carbon was supplied by Pica, Vierzon, France. Polyacrylonitrile (PAN) Derived Carbon. Polyacrylonitrile (PAN) powder supplied by Sigma Aldrich Company Ltd. was pretreated in air with a flow rate of 10 mL min-1 at 473 K for 1 h. The pretreated PAN was heated at a rate of 3 K min-1 in argon with a flow rate of 20 mL min-1 to 1173 K. The argon flow was then changed to carbon dioxide with a flow rate of 20 mL min-1, and the char was gasified at 1173 K for 4 h before switching back to argon and cooling to room temperature. This gave a carbon, designated code PANC, which had a burnoff of ∼60 wt %. Inorganic Materials Used. Ca(NO3)2‚4H2O (99.997%), Cd(NO3)2‚4H2O (99.999%), Pb(NO3)2 (99.99%), Cu(NO3)2‚2.5H2O (99.99%), and Hg(NO3)2‚H2O (99.99%) were supplied by SigmaAldrich Company Ltd., U.K. 2.2. Treatment Procedures. The two suites of carbons were based on (i) a low nitrogen content (∼1.0 wt % daf) coconut shell carbon (G) and (ii) a high nitrogen content (∼8 wt % daf) carbon derived from polyacrylonitrile (PAN). Both the PAN and coconut shell derived carbons were subjected to low temperature chemical oxidation followed by heat treatment procedures to systematically vary the type and concentration of oxygen functional groups over a wide range of concentrations. Chemical Oxidation Treatment. Carbons G and PANC were refluxed in 7.5 M HNO3 solution for 48 h. The oxidized carbons were Soxhlet extracted with water, until the pH of the aqueous extract was constant, to remove residual HNO3 and any soluble materials. The samples were dried in a vacuum at 348 K. The resultant oxidized carbons derived from PANC and G were designated sample codes PANCN and GN, respectively. Heat Treatment Procedures. Carbons PANCN and GN were heat treated to provide a suite of carbons where the oxygen functional groups of various thermal stabilities were varied progressively. The carbon samples were heat treated under argon (flow rate 20 mL min-1) at 3 K min-1 and held at the heat treatment temperature (HTT) for 1 h before cooling under argon to room temperature. The resultant carbons were designated using the code of the original carbon followed by a number to indicate the HTT in kelvins; for example, GN573 represents GN heat treated to 573 K and held at the HTT for 1 h and, similarly, PANCN573 represents PANCN heat treated to 573 K and held at the HTT for 1 h. 2.3. Chemical Analyses. Proximate analyses were performed by the method described previously.7 Elemental analyses were determined by Microanalytical Services, Oakhampton, U.K. (13) Gabaldon, C.; Marzal, P.; Ferrer, J.; Seco, A. Water Res. 1996, 30, 3050.

Langmuir, Vol. 21, No. 9, 2005 3893 2.4. Porous Structure Characterization. The micropore and total pore volumes of activated carbons used were evaluated by the adsorption of CO2 at 273 K and N2 at 77 K, respectively. The measurements were carried out using a Hiden Analytical Ltd. intelligent gravimetric analyzer.14 The micropore volume was obtained from the carbon dioxide adsorption data (p/p° ) 0-0.03) by extrapolation of the Dubinin-Radushkevich equation to p/p° ) 1. An adsorbed phase density (FCO2) of 1.023 g cm-3 was used in the calculation. The total pore volume was obtained from the uptake of N2 at p/p° ) 1 obtained using the Langmuir equation, using a density of FN2 ) 0.8081 g cm-3 for adsorbed nitrogen. 2.5. Functional Group Characterization. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra of the activated carbons were recorded using a Nicolet 20-PC FTIR spectrophotometer with a DTGS detector. The FTIR spectra were measured using KBr disks containing 0.5 wt % finely ground carbon samples. The spectral resolution was 4 cm-1 in the range 500-4000 cm-1. Acid/Base Titrations. The amphoteric characteristics of the functionalized activated carbons were characterized by the selective acid/base neutralization method.15 Aliquots of 0.2 g each activated carbon were reacted with 25 mL of 0.1 N NaOH, 0.1 N Na2CO3, 0.1 N NaHCO3, and 0.1 N HCl for 72 h. Back-titration was carried out using either HCl (0.1 N) or NaOH (0.1 N) to neutralize excess acid and bases for determining acid/base consumptions by the activated carbons. 2.6. pH at the Point of Zero Charge (pHPZC). The pH values at the point of zero charge (pHPZC) of the carbon samples were measured using the pH drift method.16 The pH of a solution of 0.01 M NaCl was adjusted between pH 2 and pH 12 by adding either HCl or NaOH. Nitrogen was bubbled through the solution at 298 K to remove dissolved carbon dioxide until the initial pH value of the solution stabilized. A total of 0.15 g of activated carbon was added to 25 mL of the solution. After the pH had stabilized (typically after 24 h), the final pH was recorded. The graphs of final pH versus initial pH were used to determine the points at which the initial pH was equal to the final pH. This point was taken as the pHPZC of the carbon. 2.7. Adsorption of Metal Ions. The Ca2+(aq), Cd2+(aq), Pb2+(aq), Cu2+(aq), and Hg2+(aq) solutions were prepared from Ca(NO3)2‚4H2O, Cd(NO3)2‚4H2O, Pb(NO3)2, Cu(NO3)2‚2.5H2O, and Hg(NO3)2‚H2O, respectively. A 0.1 g portion of activated carbon (0.7-1.7 mm) was added to 50 mL of the aqueous solution, and the solution was allowed to equilibrate for 48 h at 298 ( 0.1 K. Kinetic experiments showed that this was sufficient time for all the systems to equilibrate fully.17 The pH was allowed to vary throughout the adsorption process. The amounts of metal ions adsorbed on the activated carbon were determined by the difference of the metal ion concentrations before and after adsorption. A Unicam 701 inductively coupled plasma (ICP) atomic emission spectrometer was used to measure the concentrations of aqueous metal ions in solution. The pH values of solutions before and after adsorption were also measured to investigate the displacement of protons from the functional groups on the activated carbon by the adsorption of metal ion species and the reaction of surface basic groups with protons.

3. Results and Discussion 3.1. Characterization of Functionalized Activated Carbons. Analytical Data. Table 1 shows the analytical data for the carbon samples used in this study. The oxygen contents of the low nitrogen content G carbon series varied from ∼3 to ∼22 wt % daf, while the oxygen contents of the high nitrogen content PAN series (7-8 wt % daf) varied from ∼2 to ∼20 wt % daf. Heat treatment up to 1073 K had a marked affect on the oxygen content but little or no effect on the nitrogen content of both series of carbons. Porous Structure Characteristics. The porous structure characteristics of the carbon materials obtained from gas (14) Benham, M. J.; Ross, D. K. Z. Phys. Chem. 1989, 163, 25. (15) Boehm, H. P. Adv. Catal. 1966, 16, 179. (16) Lopez-Ramon, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMarin, F. Carbon 1999, 37, 1215. (17) Xiao, B.; Thomas, K. M. Langmuir 2004, 20, 4566.

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Table 1. Proximate and Elemental Analyses for the Carbons Used in This Studya proximate analysis carbon sample

V/wt % daf

G GN GN573 GN673 GN773 GN873 GN1073

4.5 33.8 29.7 19.7 17.9 13.2 5.2

PANC PANCN PANCN573 PANCN673 PANCN873

1.0 26.6 18.7 16.3 10.3

A/wt % db

elemental analysis C/wt % H/wt % N/wt % O/wt % daf daf daf daf

G Carbon Series 2.7 96.22 0.5 76.34 0.2 79.45 0.9 83.94 0.8 85.64 0.7 89.28 0.8 94.03

0.42 0.89 0.74 0.63 0.71 0.52 0.49

0.28 1.05 1.07 1.05 1.10 1.17 1.24

2.95 22.36 17.21 13.65 12.87 9.00 3.56

PAN Carbon Series 0.7 89.28 0.37 0.5 72.66 0.99 0.4 75.55 0.85 0.4 77.07 0.95 0.3 82.33 0.93

8.19 7.55 7.94 8.16 8.70

2.18 19.75 14.22 13.40 7.99

a V, volatile matter; daf, dry ash free; A, ash content; db, dry basis.

Table 2. Pore Structure Characterization Data for the Activated Carbons Used in This Study Determined by the Adsorption of Carbon Dioxide at 273 K and Nitrogen at 77 Ka pore volume carbon sample

VCO2b/ VN2c/ Smaxd/ E0e/ cm3 g-1 cm3 g-1 VCO2/VN2 m2 g-1 kJ mol-1

Xf/ nm

G GN GN573 GN673 GN773 GN873 GN1073

0.334 0.262 0.258 0.235 0.234 0.225 0.260

G Carbon Series 0.484 0.690 0.366 0.716 0.413 0.625 0.443 0.530 0.448 0.522 0.455 0.495 0.593 0.385

898 704 693 631 629 604 699

27.14 24.96 24.37 24.23 25.16 24.01 26.53

0.42 0.47 0.49 0.49 0.47 0.50 0.44


0.153 0.112 0.141 0.127 0.148

PAN Carbon Series 0.204 0.750 0.181 0.619 0.213 0.662 0.216 0.588 0.236 0.627

411 301 379 341 397

33.22 32.57 29.55 35.43 32.27

0.29 0.31 0.37 0.25 0.31

Figure 1. FTIR spectra of the carbon G series: (a) GN (before Soxhlet extraction); (b) GN; (c) GN573; (d) GN673; (e) GN773; (f) GN873; (g) GN1073; (h) G.

a Density of the adsorbed phase (g cm-3): nitrogen, 0.8081; carbon dioxide, 1.023. b Obtained from an intercept of the DR plot for CO2 adsorption at 273 K. c Obtained from the Langmuir model at p/p° ) 1 for N2 adsorption at 77 K. d Maximum DR surface area (CO2 at 273 K; CO2 cross-sectional area, 1.156 × 105 m2 mol-1). e Characteristic energy in DR equation (CO at 273 K, β ) 0.35). 2 f X is the mean radius of microporosity from the Dubinin Stoeckli method of CO2 adsorption data at 273 K.18

adsorption studies are given in Table 2. The total pore and micropore volumes were obtained from nitrogen (77 K) and carbon dioxide (273 K) adsorption, respectively. Neither the total pore volumes nor the micropore volumes change markedly with heat treatment within a given series. The characteristic energies (E0) and mean pore radii (X) obtained by the method of Dubinin et al.17 show that the carbons in the G series are very similar. Similar observations were obtained for the PAN series of carbons. However, the PAN carbon series has a narrower average pore radius than the G carbon series. The porous structures of both the G and PAN series of carbons may be described as nanoporous. Therefore, the surface functional groups in a given series can be studied without having marked effects due to changes in porous structure. Surface Functional Group Characterization. FTIR. Figures 1 and 2 show the FTIR spectra of the G and PAN series of carbons, respectively. Nitric acid oxidation produces new peaks and modifies the intensities

Figure 2. FTIR spectra of the carbon PAN series: (a) PANCN; (b) PANCN573; (c) PANCN673; (d) PANCN873; (e) PANC.

of existing peaks in original carbons G and PANC. The bands at 1717 cm-1 in GN and PANCN are characteristic of CdO in the carboxylic acid group. Strong bands at


∼3430, ∼1576, and ∼1200 cm-1 are observed in all the carbons except G. These bands are assigned to (i) the Os H stretching vibration due to hydroxyl groups and adsorbed water; (ii) aromatic ring stretching vibrations highly conjugated with CdO; and (iii) the CsO stretching and OsH bending modes of alcoholic, phenolic, and carboxylic groups, respectively.19,20 Heat treatment decomposes the oxygen functional groups in the carbons, resulting in a decrease in the intensity of 1717 cm-1. The band shifts from 1717 to 1734 cm-1 with increasing heat treatment temperature for the G series of carbons, and this may be due to decomposition of the carboxylic acid groups and partial intermolecular rearrangements to form lactone groups.21 This trend is not observed for the PAN series of carbons; the 1734 cm-1 band present in GN673 (see Figure 1d) is not observed in PANCN673 (see Figure 2c). This is possibly due to the influence of the high nitrogen content in the PAN series on the decomposition reactions of the surface oxygen functional groups. Comparison of the FTIR spectra of PANC and G shows that the latter is weaker and less well defined. The spectrum of PANC is quite similar to PANCN and GN which have very large oxygen contents (∼20 wt %). The main difference is the presence of a band at 1717 cm-1 in PANCN and GN which is absent in PANC. The broad band at 1200 cm-1 in PANCN and GN shifts to lower energy in PANC and is virtually absent in G. The results suggest that the infrared bands at 1576 and ∼1150 cm-1 are due, in part, to nitrogen functional groups in PANC and these overlap with the bands in oxidized carbons. Infrared bands are found at ∼1512 cm-1 (pyridinic), 1541 cm-1 (pyridonic), and 1570 cm-1 (indolic/pyrrolic) in organic compounds. The surface nitrogen groups present in the PAN series of carbons have broad absorptions due to the range of structural environments and are obscured by the strong 1576 cm-1 band, which is present in both the low nitrogen G and high nitrogen PAN series of oxidized carbons. The infrared spectra for the G and PANC series of carbons with the same HTT are similar. It is apparent that the FTIR spectra of both the G and PANC series are dominated by carbon oxygen vibrational bands. X-ray Absorption Near Edge Structure Spectroscopy (XANES). The surface nitrogen functional groups may be identified by XANES nitrogen K edge spectra.1,2,22,23 XANES studies have shown that PANCN and GN have pyridine N-oxide surface species incorporated by nitric acid oxidation.2,22 Heat treatment of PANCN decomposes the surface groups progressively, and they were absent in PANCN673. Pyridinic, pyridone, and pyrrolic functional groups were present in all of the carbons in the PAN series.22 Nitrogen functional groups are more thermally stable than oxygen functional groups. Although the PAN series of oxidized carbons has oxygen functionalities similar to those observed in the G series of oxidized carbons, the decomposition of oxygen functional groups with heat treatment does not markedly influence the forms of nitrogen functionalities present in carbons.22 The conversion of pyrrolic and pyridone nitrogen functional(18) Dubinin, M. M.; Stoeckli, H. F. J. Colloid Interface Sci. 1980, 75, 34. (19) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21; p 147. (20) Starsinic, M.; Taylor, R. L.; Walker, J. P. L.; Painter, P. C. Carbon 1983, 21, 69. (21) Chambrion, P.; Orikasa, H.; Suzuki, T.; Kyotani, T.; Tomita, A. Fuel 1997, 76, 493. (22) Xiao, B.; Boudou, J. P.; Thomas, K. M. Langmuir, published online Mar 11, http://dx.doi.org/10.1021/la0472495. (23) Zhu, Q.; Money, S. L.; Russell, A. E.; Thomas, K. M. Langmuir 1997, 13, 2149.

Langmuir, Vol. 21, No. 9, 2005 3895 Table 3. Acid/Base Titration and pHPZC of the Activated Carbons Used in This Study HCl

titration/mequiv g-1 NaOH Na2CO3 NaHCO3

G GN GN573 GN673 GN773 GN873 GN1073

0.61 0.02 0.00 0.00 0.10 0.19 0.57

G Carbon Series 0.08 0.00 5.44 3.58 3.49 2.32 2.81 1.65 2.30 1.17 1.44 0.56 0.51 0.05

0.00 2.45 1.56 1.02 0.77 0.35 0.00

8.14 2.53 3.04 3.30 3.51 4.15 7.85


0.72 0.00 0.00 0.00 0.28

PAN Carbon Series 0.37 0.00 3.39 2.33 2.23 1.27 1.66 0.92 0.79 0.22

0.00 1.78 0.86 0.43 0.00

8.22 2.55 3.05 3.39 5.76

carbon sample

pHPZC

ities to the more stable pyridinic and quaternary forms takes place with increasing heat treatment temperature.24 Acid/Base Titration. The type and concentration of functional groups give the amphoteric characteristics to activated carbons in aqueous solution. The basic properties are ascribed to the presence of basic oxygen and nitrogen functional groups and the π-electron system of the basal planes of carbon binding protons from aqueous solution.25,26 The acidic oxygen functional groups, incorporated by nitric acid oxidation, give Brønsted acid characteristics to functionalized activated carbon surfaces, resembling those of simple compounds. Table 3 shows that all of the carbons in both the G and PAN series react with both NaOH and HCl, showing the amphoteric character of the carbons. Carbons G and PANC do not react with sodium carbonate and sodium bicarbonate, indicating the absence of strongly acidic surface groups. Carbons GN and PANCN contain a large amount of carboxylic functional groups, which play an important role in the adsorption of metal cation species from aqueous solution. The concentration of lactone and carboxylic groups as determined by titration methods decreases with increasing HTT for the G series of carbons. The functional group concentrations decrease linearly with decreasing oxygen contents for both the G and PAN carbon series, as shown in parts a and b of Figure 3, respectively. The order of thermal stability is the following: carboxylic > lactone > phenol. Comparison of the G and PAN series shows that the titration values for bases are lower for the latter while the oxygen contents are similar for a given HTT. This difference can be attributed to the presence of nitrogen functional groups and the amphoteric character of carbon surfaces rather than differences in the amounts of specific oxygen functional groups. Therefore, the absolute values of the oxygen surface functional group concentrations obtained for the PAN series are not directly comparable with the G series and should only be considered in relative terms. pH at the Point of Zero Charge. The point of zero charge (pHPZC) of carbon depends on the chemical and electronic properties of the carbon surface functional groups. At the point of zero charge, the carbon surface charges are effectively neutralized so that the net surface charge is zero.27,28 Table 3 gives the pHPZC values for both the G and (24) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (25) Leon y Leon, C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797. (26) Boehm, H. P. Carbon 1994, 32, 759. (27) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990, 28, 369.

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(aq) adsorption on the G and PAN series of carbons. All the isotherms are type I in the liquid phase adsorption classification scheme29 and follow the Langmuir isotherm30 which is described by the following equation:

n)

Figure 3. Variation of functional group concentrations determined by titration methods with oxygen content: (a) G series; (b) PAN series.

PAN series of carbons. The low pHPZC values of GN (2.53) and PANCN (2.55) are attributed to the acidic oxygen functional groups (carboxylic, lactone, and phenol) incorporated on the carbon surfaces by nitric acid oxidation. Decomposition of the thermally labile oxygen functional groups on carbon surfaces by heat treatment increases the pHPZC, and the values for the G and PAN series are very similar for heat treatment up to 673 K (see Table 3). The effect of nitrogen functional groups on amphoteric character becomes apparent for heat treatment to higher temperatures; for example, GN873 has a pHPZC of 4.15, whereas PANCN873 has a pHPZC of 5.76. The high pHPZC values of G (8.14) and PANC (8.22) are due to the basic oxygen groups (chromene and pyrone) and basic nitrogen groups (pyridinic, pyridone, and pyrrolic groups) in PANC and the π-electron system of the basal planes of carbons. Therefore, the H-type carbons G and PANC readily adsorb protons from solution. In contrast, carbons GN and PANCN release protons to form net negative surface charges, which tend to adsorb metal cations. There is competition between protons and metal ions in solution for the available surface sites. 3.2. Adsorption of Aqueous Metal Ion Species in Functionalized Nanoporous Activated Carbons. Isotherms of Aqueous Metal Ion Species. Figures 4-6 show the isotherms of aqueous Cd2+(aq), Pb2+(aq), and Hg2+(28) Lau, A. C.; Furlong, D. N.; Healy, T. W.; Grieser, F. Colloids Surf. 1986, 18, 93.

nmKC 1 + KC

(1)

where n is the amount adsorbed (mmol g-1), nm is the maximum amount adsorbed (mmol g-1), K is the adsorption constant (L mmol-1), and C is the solution concentration (mM). The parameters derived from Langmuir isotherms are listed in Table 4. The maximum amounts of metal ions adsorbed (nm) for GN and PANCN are higher than those of unoxidized carbons G and PANC. This is ascribed to large amounts of acidic oxygen functional groups incorporated in these carbons. The decrease in nm with decreasing oxygen content and increasing heat treatment temperature for the G series is due to the decomposition of surface oxygen functional groups. Figure 7 shows the variation of nm for the adsorption of Cd2+(aq) on both the G and PAN series of carbons. The nm value for Cd2+(aq) adsorption is higher for PANC than G. Both carbons have similar low oxygen contents, but the nitrogen content of PANC is much higher. It is evident that the nitrogen groups influence the surface chemistry and coordinate with M2+(aq) species. Previous studies showed that the amount adsorbed of Cd2+(aq) species increased with increasing nitrogen contents for carbons with similar oxygen contents and the adsorption was strongly pH dependent.1 The Langmuir adsorption constant (K) values for metal ion adsorption species for carbons in the G series decrease with increasing HTT and hence decreasing oxygen contents. This indicates that the oxygen functional groups incorporated have high affinities for metal ion species. This trend contrasts with that observed for the PAN series of carbons, where K has a range of 6.18-8.18 L mmol-1 and is unchanged within experimental error. This difference is attributed to the presence of nitrogen functional groups in the PAN series of carbons. Hydrolysis may influence the adsorption of metal ions on a given carbon due to the different sizes and accessibilities of the species to functionalized porous structures.16 Cd2+(aq) is the main species at pH < 7; Pb2+(aq) and some Pb(OH)+(aq) may be present in a solution of pH 4-5; and a mixture of Hg2+(aq), Hg(OH)+(aq), and Hg(OH)2(aq) exists at pH 2-3.17 The sizes of the hydrated species are in the following order: M2+(aq) > M(OH)+(aq) > M(OH)2(aq). The maximum amount adsorbed (nm) for a given carbon follows the order Hg2+(aq) > Cu2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq) for GN, while the order was Hg2+(aq) > Pb2+(aq) > Cd2+(aq) for PANCN. This indicates that the accessibility of species to the porous structure is important. The adsorption of Hg2+(aq) on carbons GN673-GN1073 and PANCN573 results in the precipitation of Hg(OH)2 in solution. The adsorption of Pb2+(aq) on GN1073 and G gave Pb(OH)2 as a precipitate. Therefore, these heat treated carbons were not used for adsorption studies of Pb2+(aq) and Hg2+(aq) species. The higher pH in the solutions clearly caused the hydrolysis of metal ion species and the precipitation of insoluble products. The lower K value (6.18 mM-1) for carbon PANCN compared with carbon GN (K ) 15.7 mM-1) indicates a (29) Lyklema, J. Fundamentals of Interface and Colloid Science: Solid-Liquid Interface; Academic Press: New York and London, 1995; Vol. II. (30) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221.


Langmuir, Vol. 21, No. 9, 2005 3897

Figure 4. (a) Isotherms for the adsorption of Cd2+(aq) on the G series of activated carbons at 298 K: GN (solid triangles pointing toward the right); GN573 ([); GN673 (solid triangles pointing toward the left); GN773 (9); GN873 (1); GN1073 (2); G (b). (b) Isotherms for the adsorption of Cd2+(aq) on the PAN series of activated carbons at 298 K: PANCN (2); PANCN573 (/); PANCN673 (1); PANCN873 (b); PANC (9).

smaller affinity of the former for Cd2+(aq). The amphoteric characteristics of carbon surfaces are influenced mainly by the acidic oxygen functional groups but also the basic nitrogen surface groups. Thermally stable nitrogen functional groups enhance the adsorption of metal ion species by a coordination mechanism at high pH rather than an electrostatic mechanism as observed for surface oxygen functional groups, at low pH. The basic nitrogen groups are protonated at low pH. Effect of Solution pH. The effect of solution pH on the adsorption of Pb2+(aq) was studied by using initial solution pH values of 5.8, 3.3, and 2.2, respectively. The results in Table 4 indicate that the amount of Pb2+(aq) adsorbed

decreases from 0.57 to 0.38 mmol g-1 with decreasing solution pH from ∼5.8 to ∼2.2. The Langmuir equilibrium constant (K) also decreases from 36.05 to 5.03 mM-1 over this pH range. This decrease in Langmuir adsorption constant (K) with decreasing initial solution pH is ascribed to (i) the competition of H+(aq) for available surface sites and (ii) the hydrolysis product Pb(OH)+(aq) concentration decreases with decreasing pH. As a comparison, the solution pH for the adsorption of Pb2+(aq) on H-type carbon G was adjusted to ∼3.2, preventing Pb(OH)2 precipitation. This gave rise to a small amount of lead adsorbed (nm ) 0.06 mmol g-1) and a lower K value due to the strong competitive adsorption of protons for carbon surface sites.

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Xiao and Thomas

Figure 5. Isotherms for the adsorption of Pb2+(aq) on the G series of activated carbons and PANCN at 298 K: GN (2); PANCN (1); GN573 (9); GN673 (solid triangles pointing toward the right); GN773 (/); GN873 (b); G (pHinitial ) 3.2) ([). Table 4. Langmuir Parameters for Adsorption Isotherms of Cd2+(aq), Pb2+(aq), Hg2+(aq), Cu2+(aq), and Ca2+(aq) Ion Species on Activated Carbons at 298 K Langmuir parameters carbon

[M2+]init/ mM

[M2+]eq/ mM

nm/ mmol g-1

Cd2+(aq)

Figure 6. Adsorption isotherms of Hg2+(aq) species on activated carbons at 298 K.

Previous studies showed that the solution pH in the range 4.1-7.0 did not affect the amount of Cd2+(aq) adsorbed on carbon G but influenced the adsorption on carbon PANC, which had a much larger metal species adsorption due to the presence of nitrogen functional groups. The lower amount of Cd2+(aq) adsorbed on carbon PANC at pH 4.1 was ascribed to protonation of the surface basic nitrogen groups.1 Proton Desorption/Adsorption. Parts a and b of Figure 8 show the variation of H+ displaced with the amount of Cd2+(aq) ions adsorbed for the G and PAN series of carbons, respectively, which have pHPZC values of e4.15. Linear relationships were observed between H+ displaced and Cd2+(aq) adsorbed for these carbons. Similar relationships were observed for other M2+(aq) carbon systems, indicating that the ion exchange ratios of surface protons and aqueous metal ion species do not vary with metal ion concentration. The displacement ratios of H+ to metal ion species adsorbed, obtained from the gradients of the straight lines, are given in Table 5. The intercepts of

0.03 ( 0.00 0.34 ( 0.01 0.25 ( 0.01 0.14 ( 0.01 0.10 ( 0.01 0.05 ( 0.00 0.04 ( 0.01 0.14 ( 0.01 0.29 ( 0.02 0.14 ( 0.01 0.10 ( 0.01 0.05 ( 0.01

K/ mM-1

Ra

4.82 ( 1.08 15.7 ( 1.18 12.6 ( 4.03 8.44 ( 2.72 7.45 ( 2.55 6.50 ( 2.34 3.84 ( 0.71 7.94 ( 1.03 6.18 ( 1.18 7.26 ( 1.53 7.55 ( 2.55 8.18 ( 3.50

0.972 0.997 0.981 0.981 0.975 0.977 0.993 0.985 0.996 0.984 0.923 0.961

G GN GN573 GN673 GN773 GN873 GN1073 PANC PANCN PANCN573 PANCN673 PANCN873

0.24-1.54 0.10-2.20 0.10-2.00 0.10-2.00 0.10-2.00 0.10-2.00 0.10-2.00 0.10-1.60 0.10-1.60 0.10-1.60 0.10-1.20 0.10-1.20

0.15-1.46 0.01-1.51 0.02-1.49 0.02-1.64 0.03-1.73 0.05-1.75 0.06-1.86 0.08-1.33 0.03-1.14 0.07-1.48 0.03-1.00 0.04-1.09

Gb GNc GNd GN GN573 GN673 GN773 GN873 GN1073e PANCN

0.09-1.79 0.09-1.79 0.09-1.79 0.10-1.90 0.20-1.47 0.20-1.47 0.20-1.18 0.20-1.57 0.20-1.57 0.20-1.96

Pb2+(aq) 0.07-0.91 0.06 ( 0.01 3.85 ( 1.93 0.915 0.02-1.13 0.38 ( 0.02 5.03 ( 0.90 0.985 0.01-0.97 0.52 ( 0.02 17.58 ( 2.30 0.992 0.01-0.73 0.57 ( 0.02 36.05 ( 4.89 0.993 0.04-0.89 0.35 ( 0.02 9.48 ( 2.55 0.986 0.07-1.16 0.20 ( 0.01 7.41 ( 1.35 0.994 0.08-0.94 0.14 ( 0.01 5.41 ( 1.73 0.979 0.09-1.34 0.12 ( 0.01 3.89 ( 0.50 0.996 0.03-1.22 0.02-0.98 0.50 ( 0.04 24.2 ( 5.12 0.990

GN GN573 PANCN

Hg2+(aq) 0.50-5.00 0.02-1.07 1.37 ( 0.08 0.55-2.76 0.22-1.29 1.16 ( 0.23 0.55-3.86 0.08-1.33 1.25 ( 0.10

7.87 ( 1.60 0.991 1.63 ( 0.72 0.980 5.85 ( 1.60 0.966

GN

Cu2+(aq) 0.10-2.09 0.02-1.11 0.63 ( 0.02

3.82 ( 0.32 0.993

GN

Ca2+(aq) 0.10-1.26 0.01-0.77 0.27 ( 0.01 19.68 ( 1.99 0.992

a R is the regression coefficient. b The pH was adjusted to 3.2. Initial solution pH ∼2.2. d Initial solution pH ∼3.3. e White precipitation.

c

these straight lines are small and consistent with the graphs passing through the origin with experimental error.


Langmuir, Vol. 21, No. 9, 2005 3899 Table 5. Relationship between Protons Displaced and Cd2+(aq), Pb2+(aq), Hg2+(aq), Cu2+(aq), and Ca2+(aq) Adsorbeda metal ion

gi

ci/mmol g-1

R

pHinit

pHadsb

Cd2+(aq) G GN GN573 GN673 GN773 GN873 GN1073 PANC PANCN PANCN573 PANCN673 PANCN873

Figure 7. Variation of the maximum amount adsorbed (nm) of Cd2+(aq) determined using the Langmuir equation with oxygen contents for the G and PAN series of activated carbons.

4.70-5.51 1.97 ( 0.08 -0.04 ( 0.02 0.993 4.65-5.69 1.17 ( 0.09 -0.01 ( 0.01 0.984 4.08-5.60 0.82 ( 0.09 0.01 ( 0.01 0.968 4.08-5.60 0.80 ( 0.08 -0.01 ( 0.01 0.975 4.08-5.60 0.13 ( 0.03 0.00 ( 0.00 0.879 4.08-5.60 4.08-5.60 4.71-5.55 1.83 ( 0.06 -0.03 ( 0.01 0.995 4.83-5.75 1.79 ( 0.07 -0.05 ( 0.01 0.994 4.82-5.32 1.44 ( 0.17 -0.04 ( 0.01 0.972 4.66-5.61 4.79-5.67 Pb2+(aq)

Gc GNd GNe GN GN573 GN673 GN773 GN873 GN1073f PANCN

1.49 ( 0.11 1.51 ( 0.05 1.47 ( 0.08 1.25 ( 0.07 1.17 ( 0.10 1.08 ( 0.03 0.41 (0.03 -0.02 ( 0.01 1.40 ( 0.08

0.02 ( 0.03 0.02 ( 0.02 -0.04 ( 0.03 -0.06 ( 0.02 0.01 ( 0.01 -0.03 ( 0.00 -0.01 ( 0.01 -0.00 ( 0.00 -0.01 ( 0.03

0.980 0.995 0.986 0.993 0.986 0.998 0.989 0.909 0.989

∼3.20 ∼2.20 ∼3.30 5.10-6.47 5.26-5.62 5.23-5.54 5.23-5.77 5.15-5.42 5.02-5.23 5.21-5.45

6.73-7.13 2.87-4.08 3.21-4.43 3.44-4.12 3.59-4.48 3.91-4.92 5.73-6.71 6.56-7.06 3.08-4.07 3.51-4.35 3.72-4.87 5.53-5.88 4.89-5.70 2.06-2.17 2.61-3.16 2.75-3.47 3.07-4.09 3.36-3.92 3.68-4.12 4.13-4.60 5.53-5.81 2.84-3.60

GN GN573

Hg2+(aq) 0.71 ( 0.03 -0.01 ( 0.03 0.994 2.07-2.95 1.97-2.85 0.63 ( 0.03 0.01 ( 0.02 0.996 3.00-3.54 2.71-3.28

GN

Cu2+(aq) 1.31 ( 0.02 -0.02 ( 0.01 0.999 ∼ 5.34

2.80-4.17

GN

Ca2+(aq) 1.87 ( 0.07 -0.03 ( 0.01 0.995 ∼6.50

3.04-3.91

Hi ) gini + ci. Hi, number of protons displaced; gi, displacing ratio of protons; ni, amount of metal ions adsorbed; ci, intercept of straight line. b pHads gives the pH range variation for the isotherm. c The pH was adjusted to 3.2. d Initial solution pH ∼2.2. e Initial solution pH ∼3.3. f White precipitation. a

Figure 8. Variation of H+ displaced with Cd2+(aq) ions adsorbed on activated carbons at 298 K: (a) G series; (b) PAN series.

The proton displacement/metal ion adsorbed ratios for carbon GN follow the order (see Table 5) Cd2+(aq) (1.97) > Ca2+(aq) (1.87) > Pb2+(aq) (1.47) > Cu2+(aq) (1.31) > Hg2+(aq) (0.71). The adsorption of Hg2+(aq), Pb2+(aq), and Cu2+(aq) ion species on activated carbons is more complicated than the adsorption of Cd2+(aq) and Ca2+(aq) because of the complex hydrolysis products in solution, which give rise to lower proton displacement ratios.

The H+/M2+ displacement/adsorption ratio decreases with increasing heat treatment temperature for both the G and PAN series of carbons due to a reduction in concentration of oxygen functional groups, in particular, the acidic carboxylic groups. However, the H+/M2+ displacement ratios for PANCN573 and PANCN673 are higher than those for GN573 and GN673. A comparison of the FTIR spectra of GN673 and PANCN673 (see Figures 1 and 2) shows that decomposition of the carboxylic functional groups is different in the carbons, with lactones being present in the former. Figure 9 shows the variation of protons adsorbed with the amount of Cd2+(aq) adsorbed on carbons G, GN1073, PANC, and PANCN873, which have pHPZC values of 8.14, 7.85, 8.22, and 5.76, respectively. In contrast to the other carbons with pHPZC values in the range 2.53-4.15, where the metal ions adsorbed displace protons on carbon surfaces, the carbons not only adsorb metal cations from solution but also adsorb H+(aq). The amount of H+(aq) adsorbed was much smaller compared with the amount of Cd2+(aq) adsorbed. At a given amount of H+(aq) adsorbed, the order of Cd2+(aq) ions adsorbed on carbons follows the order PANC > PANCN873 > GN1073 > G. The adsorption of Cd2+(aq) is highest for the high nitrogen content PAN carbons. This order is related to the nitrogen and oxygen contents of the carbons and the amphoteric character of the carbon surfaces. Competitive adsorption exists between H+(aq) ions and Cd2+(aq) ions, and the protonation of basic functional groups reduces the sites available for the adsorption of metal ions species.

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Xiao and Thomas Table 6. Parameters Obtained from the Nonideal Competitive Adsorption Model of Koopal et al.31 for Aqueous Metal Ions and Protons in Solutiona carbon

Figure 9. Variation of H+(aq) adsorbed with Cd2+(aq) adsorbed for activated carbons at 298 K: G (9); GN1073 (0); PANCN873 (4); PANC (O).

Competitive Adsorption between Protons and Metal Ions. Metal ion adsorption on activated carbon from aqueous solution is often treated as single ion adsorption. However, it should be considered as competitive adsorption between metal ions and protons. The nonideal competitive adsorption (NICA) model, based on a Langmuir-Freundlich (LF)type expression developed by Koopal et al.,31 was used for analyzing competitive binding of metal ions to complex heterogeneous solids. The expression derived from the model is as follows:

θi,t )

(K ˜ ici)ni

∑i(K˜ ici)n

i

×

∑i(K˜ ici)n )p 1 + (∑i(K ˜ ici)n )p (

i

i

(2)

where θi,t is the overall coverage of a certain component ˜ i is i, ci is the concentration of component i in solution, K the median value of the adsorbent-adsorbate affinity distribution of component i, and ni and p are exponents which can take values in the range 0-1 and describe the widths of the distribution on heterogeneous surfaces. The NICA equation can be used to describe competitive multicomponent adsorption on surfaces for which the single-component adsorption of component i follows a LFtype equation. When p ) 1, the surface is considered to be homogeneous. For a two-component system (A/B), the adsorption of component A can be expressed as follows:

θA,t )

(K ˜ AcA)R (K ˜ AcA)R + (K ˜ BcB)

× β

˜ BcB)β)p ((K ˜ AcA)R + (K 1 + ((K ˜ AcA)R + (K ˜ BcB)β)p

(3)

˜ B are the adsorbent-adsorbate affinity coefK ˜ A and K ficients, and cA and cB are the concentrations of A and B in solution. The exponents R and β are related to the overall exponents for the distribution widths for the singlecomponent case (a and b, respectively) by the equations a ) Rp and b ) βp. The exponent p is a measure of the width of the intrinsic surface heterogeneity. In the case of a homogeneous surface, R ) β ) p ) 1 and this represents complete ideality. The adsorption of A and B is simplified to a two-component Langmuir isotherm. In the case of local nonideality for a homogeneous surface, p ) 1, eq 2 becomes the competitive Henderson-Hasselbalch equation with R and β expressing the cooperativity of the (31) Koopal, L. K.; Van Riemsdijk, W. H.; De Wit, J. C. M.; Benedetti, M. F. J. Colloid Interface Sci. 1994, 166, 51.

nm/mmol g-1

K ˜ M/mM-1

K ˜ H/mM-1

R

707 ( 702 0.2 ( 0.3 1.9 ( 1.5 3.0 ( 1.1 3.4 ( 1.8 5.2 ( 4.8 161 ( 272 1610 ( 550 2.0 ( 1.3 5.3 ( 1.0 9.6 ( 3.1 110 ( 647

0.991 0.998 0.998 0.986 0.975 0.977 0.987 0.996 0.986 0.975 0.954 0.988

G GN GN573 GN673 GN773 GN873 GN1073 PANC PANCN PANCN573 PANCN673 PANCN873

0.03 ( 0.00 0.35 ( 0.01 0.26 ( 0.01 0.16 ( 0.02 0.11 ( 0.03 0.05 ( 0.02 0.04 ( 0.01 0.14 ( 0.02 0.33 ( 0.07 0.15 ( 0.05 0.12 ( 0.07 0.05 ( 0.01

Cd2+(aq) 4.4 ( 1.6 17.1 ( 2.0 10.3 ( 2.2 9.4 ( 8.1 7.3 ( 5.4 6.3 ( 3.67 4.7 ( 1.3 8.8 ( 3.9 7.4 ( 2.6 9.0 ( 3.4 8.1 ( 5.3 7.4 ( 6.8

Gb GNc GNd GN GN573 GN673 GN773 GN873 PANCN

0.09 ( 0.01 0.41 ( 0.03 0.52 ( 0.04 0.58 ( 0.03 0.38 ( 0.07 0.22 ( 0.02 0.16 ( 0.02 0.12 ( 0.01 0.53 ( 0.03

Pb2+(aq) 3.0 ( 0.2 6.8 ( 0.4 16.6 ( 1.6 41.6 ( 15.0 9.7 ( 3.3 8.3 ( 3.2 7.3 ( 3.6 4.5 ( 0.9 25.8 ( 2.0

3.6 ( 0.4 0.10 ( 0.02 0.18 ( 0.07 0.32 ( 0.74 1.36 ( 1.28 1.70 ( 0.40 2.41 ( 0.72 3.00 ( 0.74 0.41 ( 0.06

0.981 0.988 0.990 0.982 0.987 0.994 0.992 0.998 0.991

GN GN573 PANCN

1.37 ( 0.08 1.96 ( 0.93 1.51 ( 0.22

Hg2+(aq) 8.9 ( 3.8 2.0( 0.9 7.4 ( 2.7

0.43 ( 0.22 0.48 ( 0.51 0.53 ( 0.10

0.988 0.963 0.967

GN

0.27 ( 0.01

Ca2+(aq) 19.4 ( 1.7

0.00 ( 0.40

0.996

The refinement with R ) β ) p )1. R is the regression coefficient. b The initial pH was adjusted to 3.2. c Initial pH 2.2. d Initial pH 3.3. a

adsorption processes of A and B, respectively. Since R and β are Cu2+(aq) > Cd2+(aq) > Ca2+(aq).32 The highest amounts of M2+(aq) adsorbed and protons displaced occur where hydrolysis in solution results in M(OH)+(aq) and M(OH)2(aq), which have smaller hydrated sizes than M2+(aq) due to their lower charges. These hydrolysis species have greater accessibility to the porous structure, thereby allowing a greater amount of adsorption. Hence, M2+(aq) species, which undergo hydrolysis to M(OH)+(aq) and M(OH)2(aq), have a higher maximum amount of metal species adsorbed and a lower displacement of protons per metal ion adsorbed.17 K ˜ H obtained from the model of Koopal et al.31 reflects the ability for activated carbon to adsorb protons in solution. A comparison of K ˜ H for the adsorption of Cd2+(aq) species on both the G and PAN series of activated carbons shows that the constant K ˜ H tends to increase with increasing HTT (see Table 5). K ˜ Cd for the adsorption of Cd2+(aq) on the G series decreases as the oxygen concentration and proton displacement ratio decrease. These trends are ascribed to the decomposition of acidic oxygen functional groups, in particular the carboxylic acid groups in activated carbon. In contrast, K ˜ Cd for the PAN series does not change so markedly with carbon heat treatment temperature, reflecting the effect of nitrogen functional groups. This is due to the amphoteric characteristics of carbon surfaces involving interaction between the nitrogen and oxygen functional groups, and there is evidence from the FTIR spectra that lactone formation in the PAN series is much less during heat treatment. Similar results were found for the adsorption of Pb2+(aq) on the G series. The (32) Bunting, J. W.; Thong, K. M. Can. J. Chem. 1970, 48, 1654.

Langmuir, Vol. 21, No. 9, 2005 3901

Figure 10. Comparison of experimental isotherm results and calculations using the nonideal competitive adsorption model of Koopal et al.31 for M2+(aq)/H+(aq) activated carbon systems in this study. Cd2+(aq): GN, GN573, GN673, GN773, GN873, GN1073, G, PANC, PANCN573, PANCN673, PANCN873. Pb2+(aq): GN, GN573, GN673, GN773,GN873, GN1073, G. Hg2+(aq): GN, GN573, PANCN.

hydrolysis products possibly present in Pb2+(aq)/H+(aq) and Hg2+(aq)/H+(aq) systems depend on pH and have a more complex influence on K ˜ H than other systems. G, GN1073, and PANC are basic carbons, while PANCN873 is slightly acidic with a pHPZC of 5.76. These carbons adsorb protons from solution and increase the equilibrium pH values, as shown in Figure 9. The other carbons studied have pHPZC values in the range 2.534.15, and protons are displaced from the carbon surface by metal ions. The K ˜ H values for the basic carbons are much higher than those observed for the carbons with acidic functional groups. This is due to the stronger interactions between basic carbon surfaces and protons in solution. The values of K ˜ Cd are in the order PANC > PANCN873 > GN1073 > G which follows the trend in metal ion adsorption in Figure 9, and this is different from the pHPZC trend. This shows the involvement of a different mechanism involving the coordination of metal ions to nitrogen functional groups. The experimental results are compared with the values calculated by eq 3, using the parameters given in Table 6, in Figure 10, for all the systems studied. Similar comparisons for individual systems are shown in the Supporting Information. The results indicate that eq 3 with R ) β ) p ) 1 provides a good description of the experimental results, indicating that all the metal ion/ adsorbent systems behave as ideal systems as though they have homogeneous surfaces. In this case, the model of Koopal et al.31 simplifies to the extended Langmuir model. It is evident that the model is widely applicable to the adsorption of metal ions on a wide range of oxygen and nitrogen functionalized activated carbons. 4. Conclusions The low nitrogen G and high nitrogen PAN series of carbons, with similar nitrogen contents and porous structures across each series but widely varying oxygen contents, were synthesized. These series of carbons allowed the adsorption of aqueous metal ions to be studied in relation to the type and concentration of surface functional groups without marked effects due to differences in porous structure. The adsorption isotherms followed the Langmuir isotherm for all metal ion/activated carbon systems studied, indicating ideal behavior. The maximum amounts adsorbed in oxidized activated carbons GN and PANCN,

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determined using the Langmuir equation, follow the order Hg2+(aq) > Cu2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq). The amount of Cd2+(aq), Pb2+(aq), and Hg2+(aq) ions adsorbed decreases with decreasing surface oxygen functional group concentration. The adsorption of aqueous metal ions on the oxidized PAN carbon series is more complex than that on the oxidized G series of carbons, because of the high concentration of both basic nitrogen and acidic oxygen functional groups present in the former. The adsorption of aqueous metal ion species, M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism with proton displacement, while the adsorption on nitrogen group sites involves a coordination mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed for both the G and PAN series of carbons with a pHPZC of e4.15 have a linear relationship. The ratio decreases with increasing heat treatment temperature and decreasing oxygen content mainly due to the decomposition of

Xiao and Thomas

carboxylic acid surface functional groups. The carbons with pHPZC values of g5.76 adsorbed both protons and metal ion species from the aqueous solutions. The protons adsorbed are 10-40% of the metal ions adsorbed, with the highest values obtained for the PAN carbons. This is attributed to the protonation of pyridinic functional groups. The adsorption of aqueous metal ions on functionalized activated carbons was analyzed using the nonideal competitive adsorption model for all M2+(aq) (M ) Ca, Cd, Hg, Pb, and Cu) and H+(aq) systems studied. This covered activated carbons which varied from acidic to basic characteristics, and all the systems behaved as though they were ideal. Supporting Information Available: Isotherms and details of model fitting to the experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. LA047135T

Adsorption of Aqueous Metal Ions on Oxygen and Nitrogen

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